The long term objective of this work is to understand the mechanisms for cardiac autorhythmicity in health and disease. The objective of this proposal is to determine the ionic mechanisms underlying cardiac pacemaking in developing heart. Cardiac arhythmias are common to all ages and frequently lead to sudden cardiac death. Most arhythmias are a result of cardiomyopathies in which there is evidence showing that embryonic protein isoforms, important for rhythmicity and contractility, are re-expressed. Spontaneous rhythmic activity begins very early in heart development as the cardiac crescent folds to form a hollow tube and begins pumping blood. The relative simplicity of the embryonic cardiac myocyte makes it useful in discerning mechanisms responsible for autorhythmicity in the embryonic and adult heart. All of the myocytes isolated from the early embryonic heart tube, including the primitive ventricle, are autorhythmic and beat spontaneously when isolated as single cells. The basis of rhythmicity in the embryonic heart is unknown. The proposed experiments will test the hypothesis that the ionic current carried by the Na/Ca exchanger is coordinated with the activity of Ca2+, K+ currents to establish the pacemaker diastolic depolarization in embryonic heart. The aims are: 1) determine whether the inward current underlying the cardiac pacemaker diastolic depolarization is carried exclusively by INaCa in the developing embryonic heart; 2) determine the contributions of Ca2+ from Ca2+ currents, the SR, and other sources to pacemaker function in heart development; and 3) determine the relationships between lNaCa, If and K+ currents in generating the diastolic depolarization in heart development. Patch clamp methods in conjunction with the use of fluorescent Ca and other ion indicators, and computer modeling will be used to determine the mechanisms of auto-rhythmicity at the cellular level in early development of the tubular heart in both mouse and chick embryos. Developmental expression of key proteins will be determined using quantitative RT-PCR and immunoflourescence confocal microscopy. The results from the completion of the proposed experiments will lead to a greater understanding of cardiac arhythmias in human heart disease and facilitate the design of treatment strategies.